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The dreaded Misfire code p0300

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    • Dec 06, 2017 3:46:47 pm
    So I just recently bought a 2008 Scion tC rs4.0 and it has a CEL on it and it can’t seem to figure out what is causing it. It has random misfire on cylinders 1 2 3 it drives fine but it does have a rough idle. I recently cleaned out throttle body and did a tuneup helped but still have the same codes. Also did a compression test and the results were 156 152 132 158
    • Dec 06, 2017 6:55:41 pm
    wvadam wrote:
    So I just recently bought a 2008 Scion tC rs4.0 and it has a CEL on it and it can’t seem to figure out what is causing it. It has random misfire on cylinders 1 2 3 it drives fine but it does have a rough idle. I recently cleaned out throttle body and did a tuneup helped but still have the same codes. Also did a compression test and the results were 156 152 132 158


    how many miles on your tC?
    • Dec 06, 2017 8:25:11 pm
    128000
    • Dec 06, 2017 10:16:14 pm
    wvadam wrote:
    128000


    that's what I thought it might be, over 100k miles. It has been a well-known story that the ignition coil packs go bad just over 100k miles. If it was a specific cylinder mis-firing then you could move that coil pack around and see if the CEL changes location of the misfire. I don't think you should just throw parts at a car. And definitely diagnose everything else by a process of elimination.

    The first step is to replace the spark plugs, the stock plug is NGK part 4589, Laser Iridium plugs. Try that first, if that doesn't work, then I'd probably check the intake for any air leaks. After that, replace the ignition coils. But the original plugs were only designed to last 100k miles. Hope that helps
    • Dec 07, 2017 6:43:48 am
    All of those has been replaced before I bout the vehiclee and misfire code says it a random misfire between 2 or more cylinders
    • Dec 07, 2017 7:05:43 am
    Meant to say I bought the vehicle with these allready replaced.
    • Dec 07, 2017 1:53:07 pm
    wvadam wrote:
    Meant to say I bought the vehicle with these allready replaced.

    well that doesn't mean they couldn't go bad, nothing is perfect.

    but.. since they were replaced, then replace the spark plugs, if you're going to tell me that they were also just done, still applies. take them out, and take a look at them, post up pics. Also use a multimeter set to ohms and test the resistance of the plugs.
    • Dec 07, 2017 4:28:25 pm
    Aight put new plugs in it still misfiring but I think it helps might just be in my head tho reset computer and CEL came back on after bout 10 miles of driving higheay
    • Dec 07, 2017 11:10:14 pm
    wvadam wrote:
    Aight put new plugs in it still misfiring but I think it helps might just be in my head tho reset computer and CEL came back on after bout 10 miles of driving higheay


    ok what spark plugs did you use? part number? brand?.. what code are you getting now with the CEL?
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